BryanMcPhail.com

Professional software development, amateur BMW tinkering, old arcade game stuff

By

Tecmo Rygar arcade pcb repair

Game played but had no sound.  The audio amp (M51516L) was getting 12V ok, but produced absolutely no hum or hiss which is a good sign the amp itself has failed.  Before replacing I tried the usual trick of attaching power speakers to the preamp outputs but this didn’t give any sound either.  The audio CPU, ROM and RAM all tested good so I moved onto the YM3526 sound chip.  The IRQ pin was pulsing which is a good sign it’s working as the CPU must program the IRQ timer for this to happen.  However, a scope on the digital audio output stream showed it wasn’t trying to play any sound.

My next theory was the main CPU wasn’t telling the audio CPU to play anything – most games of this era use an 8 bit latch to pass sound commands.  The main CPU writes the audio command into the latch then signals an interrupt on the audio CPU to signal command data is ready.  A logic probe on the audio CPU NMI line showed it pulsing whenever a sound should play (such as coin insertion) so I took some guesses at what chip the latch was and found a nearby Fujitsu LS374 where the clock also pulsed on coin insertion (main CPU writing the command) as well as output enable (audio CPU reading the command).   Sound worked with a known good LS374 piggybacked over the top, so the Fujitsu chip had failed and was likely passing 0xff (all 8 bits high) for all audio commands.

After the LS374 and M51516L were replaced everything worked again.

_ry

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>